1. Field of the Invention
The present invention relates to a method of fabricating an ultra-small condenser microphone using a semiconductor process technology.
2. Description of the Related Art
An electret condenser microphone (ECM) is an acousto-electric transducer wherein an electret film having a semi-permanent electric polarity is formed by electretizing, and a DC (direct current) bias voltage is not needed to be applied to both electrodes of a condenser. An electret film is formed by electrically charging a dielectric film and fixing charges in the dielectric film so that a potential difference is generated between both electrodes by an electric field occurred by the fixed charges. Hereinafter, to fix charges in the dielectric film is referred to as ‘electretization’ and an amount of fixed charges is referred to as ‘an amount of deposited charges’.
In recent years, ultra-small condenser microphones have been fabricated by processing silicon substrates utilizing a micro-processing technology for semiconductor integrated circuits. Such ultra-small condenser microphones have received attention as micro-electro-mechanical system (MEMS) microphones (hereinafter referred to as MEMS microphones). MEMS microphones are incorporatedly formed on a silicon substrate using a semiconductor process technology, so that it is impossible that a dielectric film alone is taken out from the microphones and separately electretized. The Japanese Patent Laid-Open Publication No. 2007-294858 discloses that a dielectric film is electretized in a state that a MEMS microphone chip formed by micro-processing a silicon wafer is mounted on a substrate for packaging or in a state of an individual MEMS microphone chip which is separated by cutting a semiconductor substrate. In this technology, a dielectric film provided in a MEMS microphone chip is electretized by applying a corona discharge at least a time to a single or several MEMS microphone chips simultaneously by a needle electrode or a wire electrode.
A conventional assembly flow of a MEMS microphone will be discussed referring to FIG. 5 as follows. First, in a wafer fabrication step S101, a wafer whereon many MEMS microphone chips are incorporatedly formed is formed utilizing a semiconductor process technology. The wafer completed the wafer fabrication step moves forward to an assembly step. In the assembly step, firstly, the wafer is stuck on an adhesive sheet fixed to a ring frame with tension in a sticking sheet step S102. Next, in a probe inspection step S103, an electrical property is measured by contacting probe needles for measurement with each MES microphone formed on the wafer in a state that the wafer is stuck on the adhesive sheet. Based on the result of the measurement, non-defective chips at the conclusion of the wafer fabrication step S101 are screened.
Successively, in a dicing step S104, a wafer is separated (diced) into individual MEMS microphone chips. After that, in an UV irradiation/pick-up step S105, adhesion of the adhesive sheet is reduced through UV (ultraviolet) light irradiation, and then a MEMS microphone chip distinguished as a non-defective in the probe inspection step S103 is picked up one by one and loaded onto a tray.
The MEMS microphone chip loaded onto the tray is transferred and held one by one at an electretization processing position in a step S106 of an electretization/inspection for amount of deposited charges. In this state, a MEMS microphone chip is electretized and the electretized chip is transferred and held to a processing position of an inspection apparatus wherein the amount of deposited charges is inspected. In this state, the amount of deposited charges in an electret film is inspected whether or not a predetermined amount of charges is deposited. Based on a result of the inspection, MEMS microphone chips are classified by a defective or a non-defective and loaded onto a metal tray respectively. The individual MEMS microphone chip classified as the non-defective is heat-treated in a state that the chip is placed on the metal tray in an annealing step S107. After that, the MEMS microphone chip completed the heat treatment is transferred from the metal tray one by one to a mounting position on a substrate for packaging and bonded thereon in a dice bonding step S108. Finally, in a modularization step S109, a wire bonding between the mounted MEMS microphone chip and the substrate for packaging is performed, and then a metal cap is put on.
As discussed above, in from the UV irradiation/pick-up step S105 to the dice bonding step S108 in the assembly process shown in FIG. 5, the diced MEMS microphone chip is transferred in a state where it is loaded on the tray from one processing step to another and is transferred and processed in a production equipment used in each processing step.